In view of the global environment, development of hybrid vehicles that include an engine and a motor as a driving force source has been being promoted. Hybrid vehicles take advantage of motor travel in a low-efficiency operating range of the engine, recovery of braking energy, which was conventionally given up as heat, and the like, and thus realize low fuel consumption and have an effect on reduction in greenhouse gas.
There is a structure of a four-wheel drive hybrid vehicle in which either front wheels or rear wheels are driven by an engine and the other wheels are driven by a motor. In a vehicle with such structure, during two-wheel drive travel only with the engine or only with the motor, when slip of the drive wheel is caused by, for instance, entering a road with a friction coefficient that is sufficiently small for the wheel to slip, i.e., a low μ road, the vehicle is controlled in general so that the non-drive wheels are driven by the other driving force source in order to travel in four-wheel drive mode for recovery of vehicle driving force (NON-PATENT LITERATURE 1).
When the drive wheel slips during two-wheel drive by the engine, the non-drive wheels are driven by the motor so as to perform four-wheel drive. Since the motor in general has an excellent responsiveness to a required driving force, vehicle driving force recovery can be performed rapidly. When the drive wheel slips during two-wheel drive by the motor, on the other hand, the other wheels are driven by the engine so as to perform four-wheel drive. Since the engine needs time for its start up, a clutch engagement operation, and torque rise, there is a problem that delay occurs before the desired engine torque is transmitted to the drive shaft and thus vehicle driving force recovery can not be performed rapidly. Since delay in vehicle driving force recovery may not just cause poor operability but also cause backward movement of the vehicle during uphill traveling, it is desirable to rapidly recover the vehicle driving force.
To address this problem, Japanese Laid Open Patent Publication No. 2006-160104 presents a rapid vehicle driving force recovery performed by providing the vehicle with a motor also on the side driven by the engine. In addition, Japanese Laid Open Patent Publication No. 2005-186756 presents a method to recover from a slip state, without using engine output, by repeating increase and decrease of motor torque when the motor driving wheel slips.